The visual experience of the uncorrected astigmat provides a unique natural experiment on the effect of chronic blur on visual development. Studies of Tohono O'odham children, 30 to 40% of whom have visually significant astigmatism, provide a unique opportunity for investigation of the effects of astigmatism. The first aim of the proposed research is to determine factors associated with spectacle wearing compliance in Tohono O'odham children, which will provide essential information for the development of future strategies to increase treatment compliance. The second aim is to determine the effect of uncorrected astigmatism on both uncorrected and corrected performance of complex and everyday visual tasks (global perception of form and motion, visual motor integration, reading), and will provide insight into how astigmatism impacts everyday functioning and whether it affects the development of the ability to perform these tasks when the child is wearing best correction. The third aim is to examine the relation between the visual experience of uncorrected astigmatic children, taking into account task-specific accommodative patterns, and the specific patterns of best-corrected visual deficits that result. These data will allow us to better understand the complex relation between visual experience and visual development. Aims 2 and 3 will also provide valuable clinical information regarding the amount of uncorrected astigmatism necessary to significantly disrupt visual performance and the amount of astigmatism necessary to disrupt normal development of visual capabilities necessary to perform simple, complex, and everyday visual tasks. The final aim is to study the stability of astigmatism in Tohono O'odham children over time, and to determine the anatomical origins of their astigmatism. The research will examine long-term change in astigmatism in over 2,000 school-age children. In addition, measurements of the posterior cornea will provide fundamental data on the origin of the internal astigmatism that often compensates for external (anterior) corneal astigmatism. RELEVANCE (See Instructions): This research will will generate important clinical information to aid in the development of guidelines for treatment and prevention of astigmatism-related amblyopia, will further our basic understanding of the influence of visual experience on visual development, and will provide the first analysis of the anatomic origins of internal astigmatism in this population.